Glutaredoxins (GRXs), also known as thioltransferases, use glutathione as a cofactor for reduction of disulfides in prokaryotes and eukaryotes. We demonstrate that the vaccinia virus O2L open reading frame encodes a functional GRX, as predicted by Johnson et al. [Johnson, G. P., Goebel, S. J., Perkus, M. E., Davis, S. W., Winslow, J. P. & Paoletti, E. (1991) Virology 181, 378-381] from sequence homology. The 12-kDa protein product of the O2L open reading frame was synthesized after viral DNA replication, coincident with a major increase in cytoplasmic glutathione-dependent thioltransferase activity. The protein was associated with purified vaccinia virions and was not released by treatment with a nonionic detergent unless dithiothreitol was added. The virion-derived protein, as well as a recombinant form expressed in Escherichia coli, exhibited thioltransferase and dehydroascorbate reductase activities indicative of a functional GRX. The postreplicative synthesis of vaccinia virus GRX and its association with virions suggest that the enzyme may have novel roles in the virus growth cycle.